I have done a search and haven't found the answers that I'm looking for. I even looked at the FAQ section. I am needing the right speedometer gears for a th 350 with 3.73 gears and a 25.6" tall tire. This is an 83 Z-28. I need to know the drive gear and the driven one to use. I have just done the swap but haven't taken it off the jackstands yet. I want to have the speedo right so I don't get a ticket.

 

A 7 tooth drive and 22 tooth driven would work. The 22T is silver, P/N 3987922. I don't have a P/N for the drive.

 

Thanks. Can I just ask the dealer for the 7 tooth gear? Surely they can figure out the part number.

 

It depends on how cooperative your dealer is.

 

You got that right! The ones around here act like you're bothering them by walking through the door. Do you know some type of formula for the gears or something? I would like to know how to figure the ratios. Is there a site?

 

It's not that hard to figure out, do a search for a post by RB83L69 containing the word SMUC. He spells it out pretty well in there.

 

It's not always that simple when it comes to speedometer gears, guys. As you may or may not be aware, THM 350 trannys used two completely different styles of gears. They are the Chevrolet built or the Buick built. Just because the bell housing bolts to a Chevy doesn't necessarily mean it's a Chevrolet built model. The size of the hole in the tail housing dictates the gears needed. Chevrolet built units originally came with a tailhousing that accepts a driven gear adapter that is 7/8t" in diameter, like a manual trans. Buick built units take an adapter that is two inches in dia.

Part # for a 7 tooth drive gear to fit the 7/8" dia sleeve is 6271537.

If your unit has the large dia sleeve, gears for a 700-R4 are used. They are a finer tooth count, with the smallest drive gear having 15 teeth. Driven gears to mate with these range from 34 teeth to 45. Two different driven gear adapters/sleeves are used with this series of gear, one for 34-39 teeth, and one for 40-45 teeth. The stem of the 40-45 tooth gears is a larger dia. to prevent use in the wrong adapter. One more thing, these adapters are not the same as ones used in a THM 400.

If you have any other questions please do not hesitate to contact me
Don.

 

Just wondering how your speedo calibration went. Never heard back from you after I sent you an e-mail with the tooth counts and part numbers for the speedo gears.

Don.

 

Don, I haven't had a chance to get the gears yet because my car has been at my buddies shop getting a cage welded in. All of my calculations figure that I need a 7 drive and 21 driven. Thank you very much for the part numbers. 94-6spd